So far white LEDs were realized by combining a blue-emitting diode with a yellow emitting phosphor. Such a combination has a poor color rendition, which, however, can be improved significantly by using a red-green-blue system (RGB). Such a system uses for example a red and blue emitter in combination with a green-emitting aluminate phosphor, like SrAl2O4:Eu or BaAl2O4:Eu, with the possible addition of Mn to Eu, whose emission maximum is around 520 nm, see U.S. Pat. No. 6,278,135. However, the position of the excitation and emission bands of theses aluminates is not optimum. They have to be excited by short UV in the range of 330 to 400 nm.
New interesting luminescent materials are α-sialon materials doped with Eu2+. They have the structure MtSi12-(m+n)Al(m+n)OnN(16-n) with M is Ca or Y or rare earth metal. The value t is given as m/val+, wherein val+ is the charge of the valence of the ion M. For example, the val+ value of M=Sr2+ is 2. Thus the charge of the whole structure is fully compensated. For further details see for example “Luminescence properties of Tb, Ce, or Eu-Doped α-Sialon Materials” by J. van Krevel et al., J. Sol. St. Chem., April 2002, p. 19-24, and also “Preparation and Luminescence Spectra of Ca and rare Earth co-doped α-SiAlON Ceramics” by R.-J. Xie et al. in J. A. Ceram. Soc. 2002, p. 1229-1234, May 2002, and further on U.S.-Pub 2002/0043926, which references all deal with Ca-Sialon of the α-Sialon type. Other types of sialon were not known at this time.